Apparatus for burning fuel



July 26, 1955 R. M. HARDGROVE APPARATUS FOR BURNING FUEL 2 Sheets-Sheet1 Filed Dec. 50, 1949 FIG] INVENTOR Mflam'grore Fag/z ATTO R N EY y 6,1955 R. M. HARDGROVE APPARATUS FOR BURNING FUEL 2 Sheets-Sheet 2 FiledDec. 30, 1949 INVENTOR 1? 0 5 flff/amgm Ve ATTORNEY United States PatentAPPARATUS FOR BURNING FUEL Ralph M. Hardgrove, Canton, Babcock & WilcoxCompany, poration of New Jersey Ohio, assignor to The Rockleigh, N. L, acor- The present invention relates in general to an apparatus forburning fuels, and more particularly to an apparatus for burningash-containing particle form solid fuels at least partially insuspension under furnace conditions insuring a high combustionefficiency and permitting the separation and removal of the fuel ashconstituents from the combustion zone in a molten condition.

In the burning of an ash-containing fuel in suspension in the furnace ofa fluid heating unit containing convection heated fluid heating tubes,for example, it is important from both the standpoint of thermalefliciency and unit availability that combustion of the combustibleconstituents be completed before the gaseous products of combustionreach any relatively closely spaced convection heated fluid heatingtubes. This is especially important when the mean or average furnacetemperature exceeds the initial deformation temperature of the ashconstituents of the fuel. Under such conditions ash particles suspendedin the gaseous products of combustion are likely to be in a stickycondition when reaching such cooler heat transfer tubes and to depositthereon to such an extent as to restrict and even close the intertubegas flow passages. Ash particles which are solidified before reachingsuch heat transfer tubes, or solidified in the intertube passages, areeither deposited on subsequent heat transfer tubes in an easilyremovable condition or are carried out of the fluid heating unit andunless collected by expensive dust collecting equipment, will foul theatmosphere. In view of the increasing efforts to minimize atmosphericpollution, there has been a constant trend to separate and recover asmuch as possible of the recoverable ash content within the combustionzone in a molten condition and or zig-zag flow path, stratification ofthe gases therein is minimized, and a highly effective turbulent mixingof the fuel particles and oxygen-containing gas will occur, whilemaintaining a relatively low gaseous pressure drop between the furnaceentrance and exit. A high thermal eificiency is insured and ashseparation facilitated by the maintenance of furnace temperatures in thefusing temperature range of the fuel ash sufficient to render the ashparticles in a fluid or molten condition. Under these conditions, thesuccessive relatively abrupt changes in direction of the stream ofburning fuel, oxygen-containing gas and products of combustion causeagglomeration of the tiny particles of ash and successive separation ofthe resultant heavier ash particles from the stream. The successivechanges in direction cause the agglomerated ash particles to deposit onthe wall surfaces defining the gas flow path, while retaining thegaseous pressure drop within practical operating limits. As aconsequence, the ash particles coming into contact with the confiningwalls of the combustion zone are in a condition suitable for adher ingthereto and forming a sticky film or layer of downwardly flowing moltenash. With the gaseous flow path thus mainly defined by surfaces having asticky film of ash thereon, unburned large or heavy fuel particlescoming into contact therewith are entrapped thereon and burned in situby the scrubbing contact of the high velocity oxygen-containing gases.Thus the fuel particles entrapped on the molten ash film are not onlyprovided with the necessary oxygen for their combustion, but are alsoretained within the combustion zone long enough to complete theircombustion. Under such furnace temperature conditions in a tortuouscombustion zone, any purely refractory wall surface defining the samewould be rapidly eroded and be commercially impracticable. Such erosionis avoided, in accordance with the present invention, and

. the heat recovery efficiency of the associated fluid heating unitincreased, by the provision of fluid heating tubes in the wall surfaceslining the flow'path within the combustion zone. Advantageously, thecombustion zone shape is such that relatively simple formations of fluidheating tubes can be used to define the same, and the tubes covered witha refractory layer proportioned to facilitate solidify the remainderbefore the gaseous products of k combustion reach any closely spacedheat transfer tubes. For the burning of pulverized bituminous coal, forexample, wet bottom or slag tap furnaces have been developed in whichthe fuel is burned in suspension under furnace temperatures such thatash collecting in the combustion zone can be removed in a fluid ormolten condition. Approximately of the recoverable ash content of thefuel can be recovered in the combustion zone under these conditions.However, the ash remaining in the a better understanding of theinvention, its operating a'dgases leaving the combustion zone is stillsufficient in amount to present a considerable operating problem inmaintaining the heat transfer surface in an eflicient heat absorbingcondition, and also to cause a considerable amount of atmosphericpollution unless collected extershaped combustion zone through which thefuel and an oxygen-containing gas, such as air, are passed under highvelocity turbulent flow conditions, resulting in an intimate mixing ofthe oxygen-containing gas and the combustible fuel constituents andrapid and complete combustion thereof. With such a high velocity flowthrough a sinuous the maintenance of the deposited ash particles in afluid or molten condition within the combustion zone and to permit theirflow under the action of gravity through the combustion zone, and thecollection and removal of the separated ash in a molten condition, i. e.as molten slag, at a point outside of the main gas flow path.Substantially all of the recoverable ash constituents are thus recoveredand the gaseous products of combustion are discharged from thecombustion zone substantially free of entrained ash particles. I

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For

vantages and specific objects attained by its use, reference should behad to the accompanying drawings and descrip tive matter in which I haveillustrated and described a preferred embodiment of my invention.

Of the drawings:

Fig. 1 is an elevation, partly in section, of a vapor generating unithaving an experimental type furnace constructed in accordance with thepresent invention;

Fig. 2 is an enlarged horizontal section taken on the line 22 of Fig. l;

Fig. 3 is an enlarged elevation of the burner nozzles viewed from theinside of the furnace along the line 33 of Fig. 1; and

Fig. 4 is an enlarged transverse line 4-4 of Fig. 1.

As shown in Fig. l, the furnace 10 of the present invensection taken onthe tion is associated with a well known type of steam generating unit14. When applied to this form of heat user, the furnace is symmetricallyarranged relative to the width of the unit and advantageously formed byboundary walls which are defined by spaced tubes connected into thecirculatory system of the steam generating unit 14. The tubes arestudded in a known manner and coated with an initially plasticrefractory material, such as plastic chrome ore, to provide a refractorylining for the furnace and supporting surfaces for the film of moltennoncombustible matter deposited thereon.

In the embodiment illustrated, the front and rear walls of the furnaceare defined by rows of refractory-faced studded upright tubes and 16,respectively, shaped to define the front and rear sides, roof and floorof an upper fuel ignition chamber 11, the flow directing walls of azig-zag or sinuous passageway in the intermediate portion 12 thereof,and the front and rear sides, roof and bottom of a lower ash receivingand heating gas outlet chamber 13. The opposite side walls of thefurnace 1 are also defined by rows of refractory faced studded uprighttubes 18 and 20 extending the full height of the furnace 10 betweenpairs of upper headers 21 and lower headers 22 arranged at oppositesides of the furnace.

The upper ignition chamber 11 is bounded by the upper portions of therows of tubes 15 and 16 to define a chamber of rectangular horizontaland vertical cross-section. The furnace roof 23 is formed by theinclined upper end portions of the front wall tubes 15, the upper endsof which are connected with the front upper drum 24 of the steamgenerating unit 14. The rear tubes 16 extend downwardly from an upperend connection with the drum 24 to a spaced position below the roof 23where they slope downwardly toward the front wall tubes 15 to form thefloor 25 of the chamber 11. The floor 25 ends at a position transverselyspaced from the front wall tubes 15 to define an outlet opening 26, asshown in Fig. 2, of generally rectangular cross-section. Below theopening 26, the rear wall tubes 16 have an irregular sinuosity invertical plane, being rearwardly and downwardly bent at an inclinationof, for example, approximately 15 to the horizontal, to a positiongenerally in vertical alignment with the rear wall of the ignitionchamber. The tubes 16 are then bent vertically downward and then slopeddownwardly and forwardly to a position generally in vertical alignmentwith the rear edge of the opening 26, and then sloped rearwardly anddownwardly to substantially duplicate the described shape of thesuperjacent projecting portion. The tubes 16 are again bent verticallydownward and then forwardly and downwardly at the bottom of theintermediate portion 12 of the furnace to a position spaced from thefront wall tubes 15 to provide a bottom outlet 27 of larger rectangularcrosssectional area than the outlet 26, opening into the lowerash-receiving chamber 13.

The portions of the front wall tubes 15 in the intermediate section 12of the furnace are bent in substantially parallel relationship to therear wall tubes 16 to define the flow-directing walls of a sinuouspassageway 28 of substantially uniform cross-sectional flow areathroughout the intermediate portion 12 of the furnace. The flowpassageway 28 is formed by a plurality of straight passageway sections28A, 28B, 28C and 28D connected in substantially end to end angularrelationship, e. g, angles of 30, so as to form a flow path having aseries of relatively abrupt changes in flow direction. With thisarrangement, the tubes 16 will define the inclined roof portions of thesections 28A and 28C and the inclined floor or hearth portions of thesections 283 and 28D, while the successive sections of the tubes 15 formthe floor portions of the sections 28A and 28C and roof portions of thesections 283 and 28D. In operation, the section 28A will receive asubstantially vertical flow of burning fuel and ash in suspension fromthe ignition chamber outlet 26 and directs the fiow in a rearwardlydirection to the adjoining section 28B. In entering and passing throughthe section 28B, the stream of burning fuel and gaseous products ofcombustion makes a short radius turn through an angle of approximatelysubstantially reversing the direction of the stream. This abruptreversal of stream fiow is repeated in successively opposite directionsin entering and passing through each of the adjoining passagewaysections 28C and 28D, discharging at the end of the latter sectiondownwardly through the outlet 27 into the chamber 13.

From the outlet 27 at the top of the ash-receiving chamber 13 the frontwall tubes 15 extend downwardly to a connection in a horizontal header3i which is connected to the lower drurn 31 of the generator 14 ashereinafter described. The front and side walls of the ashreceivingchamber 13 are defined by upright lower portions of the rows of tubes15, 1S and 20. The rear wall tubes 16 are alternately bent to differentdegrees to form two rows of bare tubes which slope downwardly andrearwardly from the opening 27 to a spaced position above the fioor ofthe chamber 13 to provide an inclined slag screen 32 through which theheating gases pass in leaving the chamber 13. Beneath the lower end ofthe slag screen 52, the lower portions of the tubes 16 define an uprightrear wall and the inclined floor of the chamber 33, with the lower endsof the tubes opening into the header 30. A row of tubes 33 are inclineddownwardly from the drum 31 to merge into the tube row 16 at the lowerend of the slag screen 32 and to connect with the header 30. The floorof the chamber 13 is provided with a slag tap hole 34 near its lower endformed by bending the forward portions of some of the floor tubes. Aplug 35 is mounted on an arm 36 so that the tap hole 34 may be closed oropened by raising and lowering the plug, respectively. The arm is movedby means of a hydraulic piston 37 which can be manually regulated orarranged for periodic movement in response to impulses transmitted by asuitable timing device (not shown). The molten ash or slag droppingthrough the slag hole falls into a tank 38 of water and is removed fromthe tank or slag pit by an upwardly inclined screw conveyor 40 forsuitable disposal.

In the illustrated embodiment of the invention, an ashbearing particleform fuel, such as pulverized coal, is delivered to the ignition chamber11 in a carrier or primary air stream through a feed pipe 41. Raw coalfrom an overhead bin 42 is drawn through the feed spout 43 into a unitpulverizer 4'4, with the air-borne pulverized coal discharging therefrominto the pipe 41. The upper end of the feed pipe 41 is divided into aplurality of horizontally elongated nozzles 46 which are arranged in aspaced horizontal row to discharge the air-borne fuel throughcorresponding burner ports in the front wall of the chamber 11. Theburner ports are formed o5- setting a portion of alternate front walltubes 15 and omitting the studs and refractory therefrom, as shown inFig. 3, to provide a plurality of vertically elongated intertube burnerports 47 in the chamber front wall. The nozzles 46 have their inner endsterminating at the center line of the tube row 15 and in contact withthe adjacent tubes. The nozzles 46 and ports 47 are generally centeredhorizontally of the upper part of the chamber 11, so that the fuel andair streams discharged therefrom are initially directed substantiallyhorizontally toward the upper rear wall of the ignition chamber 11. Anexternal air housing 48 encloses the ports 47 and sur rounds the nozzles46 so that superatrnospheric pressurc air delivered to the housing froman external source will pass through the ports 47 both above and belowthe nozzles 46 to provide secondary combustion air for the fuel. Undercertain conditions of operation, it may be desirable to divide therequired amount of secondary air to the furnace. For this purpose, oneor more valved air conduits 49 extend from the bottom of the air housing48 and discharge between adjacent tubes into the passageway section2813, as shown in Fig. l.

Preheating of the secondary combustion air for the furnace of thepresent invention is desirable, although not essential. As shown in Fig.1, thesecondary air delivered to the burner ports 47 is preheated to ahigh temperature in a recuperative type air heater 50 mounted above thesteam generator. High temperature combustion gases are withdrawn fromthe generator setting before they enter the tube banks through a duct 51and passed through a plurality of groups of tubes 52 within the heaterto discharge into the stack 53. Air is delivered to the heater 50through the air duct 54 from a forced draft fan 55, passed over thetubes 52 of the air heater and delivered through the discharge duct 56to the air housing 48. With the described arrangement, air temperaturesas high as 870 F. have been obtained and the preheated air used forcombustion purposes in the described process. A valved by-passconnection 57 between the air inlet portion of the heater and thedischarge duct 56 permits a proportioning of cold and hot air toregulate the secondary air temperature delivered to the air housing 48.

In starting up, a torch 17, for example, an oil burner, is used topreheat the chamber 11. Thereafter the pulverized coal is delivered withits carrier air through the pipe 41 and the nozzles 46 to the ignitionchamber 11 where it is ignited from the torch flame. When the coal flamehas become stabilized, the use of the torch 17 is discontinued.

The pulverized coal-primary air streams entering the ignition chamber 11rapidly mix with the superjacent and subjacent secondary combustion airstreams, with most of the finer coal particles burning during theirmovement through the U-shaped path in the ignition chamber leading tothe outlet 26. The ash residue of the burned coal particles is initiallyreleased in a substantially molten state, and some of the globules ofash and other suspended solids are forced outwardly of the stream ofburning fuel and gases largely by reason of the reversal in thedirection of flow thereof in the ignition chamber due to the relativeposition of the burner nozzles 46 and the outlet 26. The fuel and airare normally supplied in sufiicient quantity to maintain the ignitionchamber at a temperature in excess of the ash fluid temperature, so thatthe globules of molten ash contacting the chamber walls will tend toadhere thereto to form a downwardly moving film of sticky ash thereon.

The burning mass or stream of gaseous products of combustion, burningcombustible matter and heated air, with its suspended globules of ash,passes through an abrupt change in flow direction ,in leaving thechamber 11 and entering the passageway 28A through the outlet 26. Thesubstantial change in flow direction intimately remixes the constituentsof the burning stream, causing agglomeration of suspended moltenashparticles and a further separation of the ash by the inertia effecton the particles and their adhesion to the walls of the passageway. Theseparated molten ash flows down the side walls and floor of thecorresponding floor or hearth por tion and either drops to the floor ofthe subjacent section or is swept around the rounded nose at the fioorend and partly down the roof section of the subjacent section before thegravitational effect causes it to drop. The straight portion of eachpassageway section causes an acceleration of any solid or liquidparticles in suspension in the burning stream, so that as the streamturns in passing from one passageway section to the subjacent section,the momentum of the particle tends to force it outwardly into contactwith the confining wall of the passageway turn. The repeated turnsthrough the described reversals in direction of the flow path of theburning stream continue the ash agglomerating and separating effect ofthe furnace. It is believed that high gas velocities through the sinuoussection of the furnace of the order of those indicated herein areessential to an effective ag glomeration of the fine molten ashparticles released in the burning of a pulverized ash-bearing solid fuelsuf ficient to cause their separation from the burning stream under thesuccessive inertia eifects imposed thereon by the successive reversalsof flow direction.

The described successive reversals in direction of the stream of burningfuel, combustion air and products of combustion are highly effective inpromoting further mixing of the partially burned and unburned fuelparticles with the combustion air in the stream, due to the turbulentflow conditions existing in the gas turning portions. The repeatedintimate mixing of unburned fuel and air during the passage of thestream through the sinuous section of the furnace contributes to highlyefficient combustion conditions, permitting the burning of low volatilepulverized solid fuels, such as pulverized petroleum coke, heretoforefound difficult to burn in suspension without the aid of an auxiliaryfuel.

The maintenance in operation of a film or layer of molten ash on theboundary walls of the ignition chamber 11 and sinuous passageway 28, andparticularly on the Wall areas in and adjacent to the gas turningportions of the sinuous passageway, also contributes to the fuel burningefiiciency of the furnace, particularly when the maximum particle sizeof the fuel exceeds that normally present in pulverized bituminous coalas is the case when a crushed or screened coal is being burned. In suchcases, the larger fuel particles tend to separate from the stream in theignition chamber and gas turning sections and to be entrapped in thefilm of molten ash on the walls of those sections. The scrubbing actionof the high velocity oxygen-containing stream relative thereto causesthe entrapped fuel particles to burn rapidly in situ, and the ashresidue joins with the ash in the molten film on those surfaces.

The combustion gases entering the ash-receiving chamber 13 of thefurnace are subjected to a final reversal in flow direction in order toreach the gas outlet from the chamber 13 across which the slag screen 32extends. This abrupt change in direction affords a final opportunity formolten ash particles to separate in the furnace. Any fine ash particlesremaining in the exiting hot gases are cooled below the fusingtemperature range of the ash as the gases pass through the slag screen32 at the rear side of the furnace 10.

In the arrangement described, the direction of gas flow through thefurnace and the flow of the molten ash wall film are generally inparallel, although relative cross-flow occurs in some parts of theapparatus. Parallel fiow is the preferred relationship of gas and ashflow direction, but the furnace may be arranged and/ or the fuel inletand slag chamber gas and slag outlets located so that the flowrelationship will be primarily of a cross-flow nature, or primarily of acounter-current flow nature, while retaining desirable ash separatingand removal and fuel burning advantages of the present invention.

By way of example and not of limitation, an experimental furnaceconstructed substantially as disclosed was operated at firing rates offrom 3000 to 6000 lbs. of pulverized bituminous coal per hour. Thebituminous coals used included a range of ash contents between 9.6 and24% volatile contents from 18 to 42%, and ash fluid temperatures underoxidizing conditions of between 2490" F. and 2700 F. In this range ofcapacities, the furnace temperatures maintained were such that moltenash was continuously tapped from the tap hole 34. The nozzle tipvelocity of the air-borne pulverized coal was from 4000 to 6000 feet perminute, while the velocity of the burning stream passing through thepassageway 28 was of the order of 5000-8000 feet per minute. With apulverized coal fineness of 70 to passing through a 200 mesh screen, 94to 99% through a 100 mesh screen, and 99.8% through a 50 mesh screen,and with excess air of the order of 10 to 20%, the dust loading of thegases leaving the furnace was found to be less than 1 pound per 1000pounds of flue gas. As a further and more specific example, pulverizedFairmont coal having a volatile content of 41.9%, and an ash content of9.6% was delivered to the furnace at a rate of 5980 pounds per hour.This coal had an ash fluid temperature under oxidizing conditions of2590 F, and was pulverized to a fineness of 70.4% passing through a 200mesh screen, 94.4% through a 100 mesh screen, and 99.8% through a 50mesh screen, for delivery to the furnace. At this capacity with asecondary air temperature of 725 F, and with 125% total air, as measuredat the air heater outlet, the dust loading at the stack was found to be.68 pound of dust per 1000 pounds of flue gases.

While the present invention is particularly effective in the combustionof, and molten ash separation in the combustion zone from, a pulverizedbituminous coal, the process and furnace disclosed can also be used toadvantage with other fuels. Relatively coarsely sized ash-bearing solidfuels can be efficiently burned with an effective ash separation in thefurnace described, whether the fuel is prepared by crushing orscreening. Such coarse particle form fuel should, however, be suppliedwith its sizing selected in view of its grindability and volatilecontent. For example, a high volatile bituminous coal which has beenprepared by crushing or screening to a A. inch top size andapproximately 10% through 200 mesh, will ordinarily contain sufficientfines for use in the furnace. A lower volatile coal should, however, beprepared with a greater percentage of fines, as for example with amaximum size of the order of /4 inch and through 200 mesh. Whenutilizing such relatively coarse particle form fuels, the molten film ofash on the walls of the combustion zone is particularly effective inentrapping coarse fuel particles so that the particles will be retainedin scrubbing contact with the oxygen containing gases to completelyconsume the combustible content of the fuel particles within thefurnace. A fluxing material, such as limestone and/or soda ash, may beadded to any of the fuels described to increase the fluidity of the ashand to lower its fusing temperature range. Similar effects can also beobtained with some ash-bearing fuels by operating the ignition chamberunder a reducing atmosphere and supplying the remainder of the secondaryair required through the pipes 49. The mixing and remixing effect of thehigh velocity flow through the tortuous refractory faced passageway ofthe furnace is effective for the combustion of low volatile low ashfuels, such as petroleum coke. The described results are accomplishedwith a relatively low pressure drop through the furnace, e. g. pressuredrops ranging from 2 to 8 in. H2O for a load range of 30006000 lbs. ofcoal per hour, and therefore a correspondingly low required capacity andpower consumption for the forced draft fan 55.

While in accordance with the provisions of the statutes I haveillustrated and described herein the best form and mode of operation ofthe invention now known to me, those skilled in the art will understandthat changes may be made in the form of the apparatus and in the fuelburning process disclosed without departing from the spirit of theinvention covered by my claims, and that certain features of myinvention may sometimes be used to advantage without a corresponding useof other features. For example, changing the cross-sectional shape ofthe passageway to reduce the ratio of furnace volume to furnaceperiphery will permit the use of fuels in a wider range of ash fusingtemperature ranges.

I claim:

1. Apparatus for burning an ash-bearing fuel comprising reversely bentrefractory coated fluid heating tubes cooperating to define a verticallyelongated tortuous passageway of substantialy uniform flow area havingsuccessive reversals in flow direction and a series of inclined moltenash-supporting surfaces, means for introducing a burning stream of fuelin gaseous suspension into the upper end of said tortuous passageway,and

means for collecting molten ash at the lower end of said tortuouspassageway substantially out of entraining contact with the discharginggaseous products of combustion.

2. Apparatus for burning an ash-bearing fuel with combustiontemperatures above the ash fusion temperature of said fuel comprisingrefractory covered fluid heating tubes cooperating to define the wallsof a vertically elongated furnace chamber, an ignition section at oneend of said elongated chamber, means for introducing a combustiblemixture of fuel in suspension and air into said igition chamber, saidelongated chamber having a tortuous passageway portion in communicationwith said ignition chamber and having successive reversals in flowdirection therein, the walls of said elongated chamber having a seriesof inclined ash supporting surfaces, and means for collecting molten ashat a point substantially out of entraining contact with the dischargingstream of gaseous products of combustion leaving said tortuouspassageway.

3. Apparatus for burning an ash-bearing fuel with combustiontemperatures above the ash fusion temperature of said fuel comprisingrefractory covered fluid heating tubes cooperating to define the wallsof a vertically elongated combustion chamber having an ignition section,means for introducing air-borne ash-bearing fuel and combustion air tosaid ignition section, a molten slag receiving section of said chamberspaced from said ignition section, and walls defining a plurality ofpassageways of reduced flow area arranged in end to end angularrelationship and extending downwardly from said igni tion section tosaid slag receiving section, said walls having a series of inclined ashsupporting surfaces.

4. Apparatus for burning an ash-bearing fuel substantially in suspensionwith temperatures in excess of ash fusion and separating the molten ashfrom the gases of combustion comprising an ignition chamber defined byrefractory lined fluid cooled walls having an outlet in the lower endthereof, a multiple tip burner arranged to discharge ash-bearing fuel inair suspension through a wall of said chamber upwardly adjacent saidoutlet, a slag receiving chamber spaced downwardly from said ignitionchamber, refractory lined fluid cooled walls defining a plurality ofpassageways arranged in series staggered relationship and extending fromthe outlet of said ignition chamber to said slag receiving chamber, saidrefractory lined fluid cooled walls having a series of inclined ashsupporting surfaces and being spaced to define a flow passageway havinga substantially uniform cross-sectional area less than thecross-sectional area of said ignition chamber.

5. Apparatus for the suspension burning of particle form ash-bearingfuel with combustion zone mean temperatures above the ash fluidtemperature and agglomerating and separating the resulting ash in amolten condition therein comprising refractory lined fluid cooled wallsdefining an elongated chamber having a fuel ignition section in one endthereof and a heating gas outlet at its opposite end, fluid cooled tubescovered by refractory material cooperating to define a substantiallyunobstructed zig-zag gas passageway and inclined molten ash receivingsurfaces between said ignition section and heating gas outlet, thecross-sectional area of said zig-zag gas passageway being approximately20 to per cent of the cross-sectional area of said fuel ignitionsection, means for introducing a combustible mixture of airborneash-bearing fuel and combustion air into said ignition section, andmeans for collecting separated ash in a molten condition from said ashreceiving surfaces at the lower end of said chamber.

6. Apparatus for burning particle form ash-bearing fuel at leastpartially in suspension with combustion zone mean temperatures above theash fusion temperature and separating the resulting ash in a moltencondition therein comprising walls formed by rows of refractory coveredvapor generating tubes defining a vertically elongated chamber having afuel ignition section in one end thereof and a heating gas outlet at itsopposite end, said refractory covered fluid cooled tubes spaced todefine a substantially unobstructed high velocity zig-zag gas passagewayand having inclined molten ash receiving surfaces between said ignitionsection and heating gas outlet, a burner arranged for introducing acombustible mixture of air-borne ash-bearing fuel and air into saidignition section, the burner and the gas inlet end of said zig-zag gaspassageway being positioned to cause the flow of fuel and air throughsaid ignition section to move through a U-shaped path, and means forcollecting separated ash in a molten condition from said ash receivingsurfaces at the lower end of said chamber.

7. Apparatus for burning particle form ash-bearing fuel in suspensionand agglomerating and separating the ash residue in a molten conditionin the combustion Zone comprising walls defining a vertically elongatedchamber having a fuel ignition section in the upper end thereof and aheating gas outlet at its lower end, sinuous refractory covered fluidcooled tubes cooperating to form a series of oppositely inclined moltenash-receiving surfaces projecting from opposite sides of said chamberand arranged to define a substantially unobstructed vertical zig-zag gaspassageway therein between said ignition 10 chamber and heating gasoutlet, means for introducing a combustible mixture of particle formash-bearing fuel and air into said ignition section and burning the samewhile in said zig-zag gas passageway, and means for collecting ashdeposited on said ash-receiving surfaces in a molten condition at thelower end of said chamber.

References Cited in the file of this patent UNlTED STATES PATENTS608,161 De Sotolongo July 26, 1898 698,190 Fenner Apr. 22, 19021,095,489 Alford May 5, 1914 1,294,730 Von Porat Feb. 18, 1919 1,651,646Trasenster Dec. 6, 1927 1,791,836 Cannon et a1. Feb. 12, 1929 1,930,566Sanders Oct. 17, 1933 2,031,551 Sorensen Feb. 18, 1936 2,128,177 CarterAug. 23, 1938 2,268,559 Bailey Jan. 6, 1942 2,275,394 Hardgrove Mar. 3,1942 2,357,303 Kerr et a1. Sept 5, 1944 FOREIGN PATENTS 518,517 GermanyFeb. 17, 1931

